The present invention relates to a water-repellent layer for fuel cell, and a fuel cell.
Hydrogen peroxide generated during operation of a fuel cell decomposes an electrolyte membrane and an ionomer of a catalyst layer. It is known that hydrogen peroxide is decomposed by addition of a cerium-containing oxide in an electrode (Japanese patent publication No. 2004-327074A). Another known technique for a fuel cell provides a cerium-containing layer on at least one surfaces of a membrane electrode assembly, in order to suppress reduction of the water content in the membrane electrode assembly. The cerium-containing layer is formed to contain a cerium-containing oxide of higher than 5 wt % but not higher than 30 wt % relative to 100 wt % of the solid content other than the cerium-containing oxide (Japanese patent publication No. 2011-113860A).
Employing a hydrogen peroxide decomposition catalyst such as cerium-containing oxide for a water-repellent layer, however, causes the following problem. The cerium-containing oxide is hardly soluble in water, so that there has been substantially no study on effect of cerium ion eluted from the cerium-containing oxide on the fuel cell. Cerium ion is, however, slightly eluted from the cerium-containing oxide in an acidic atmosphere in the electrode. The eluted cerium ion (Ce3+) is adsorbed to the sulfonic acid group (—SO3−) included in the electrolyte membrane and the ionomer of the catalyst layer to interfere with proton conduction. During the long time use of the fuel cell, adsorption of the cerium ion to the sulfonic acid group proceeds to cause reduction of the power generation performance of the fuel cell. The hydrogen peroxide decomposition catalyst is not limited to the cerium-containing oxide but may be a single element selected from an element group consisting of transition metal elements and rare earth metal elements and/or a compound including such a metal element. In the application using such a metal element or metal element-containing compound as the hydrogen peroxide decomposition catalyst, elution of the metal ion having positive charge from the hydrogen peroxide decomposition catalyst is adsorbed to the sulfonic acid group included in the electrolyte membrane and the ionomer of the catalyst layer to interfere with proton conduction. The less exposure of the hydrogen peroxide decomposition catalyst to water leads to the less elution of the metal ion. Using a water-repellent material suppresses exposure of the hydrogen peroxide decomposition catalyst to water and thereby suppresses elution of the metal ion from the hydrogen peroxide decomposition catalyst. In the case where the content of the hydrogen peroxide decomposition catalyst is excessive relative to the content of the water-repellent material, however, the water-repellent material is unlikely to sufficiently suppress exposure of the hydrogen peroxide decomposition catalyst to water.
In order to solve at least part of the above problems, the invention may be implemented by any of the following aspects.